A drill chuck normally has a chuck body surrounded by a sleeve that can be rotated relative to the chuck body in one direction to move the jaws of the chuck radially toward one another and in the opposite direction to move the jaws apart. In order to be able to apply quite some torque to this sleeve it is standard practice to form this sleeve at one end with an array of axially directed teeth and to form the chuck body adjacent these teeth with one or more pilot holes. A chuck key has a key body having a front end from which extends a pilot pin insertable into any of these pilot holes. This front end of the key body is formed with an array of teeth surrounding the pilot pin so that when the pilot pin is inserted in a pilot hole the teeth of the key mesh with the teeth of the sleeve of the chuck. A crosspiece is provided on the rear end of the chuck body so that the user can then rotate the key and exert considerable force between the chuck body and the sleeve.
A fairly common and relatively dangerous accident can occur when the chuck key is left inserted in the chuck. As the drill is started up the key will be thrown radially outwardly, potentially seriously injuring the user of the tool or a bystander.
Accordingly it has been suggested, as for example in U.S. Pat. No. 4,111,079, to provide means which insure automatic disengagement of the chuck key from the chuck when it is released by the user. In the simplest form the key is provided with a backwardly resiliently deflectable arm or spring element that is pushed backward, with concomitant elastic deformation, when the pilot pin is inserted in the hole in the chuck, but which is sufficiently springy to push the pilot pin out of the pilot hole when no axial forward force is exerted by the user on the key relative to the chuck.
It is also known to make the pilot pin axially displaceable in the chuck body. A spring urges this pin into an inner position in which it is retracted back into the chuck body. The cross-piece that is held by the user chuck is connected to the pin so that the pin can be pushed forwardly relative to the sleeve-shaped key body surrounding it. Thus the user must push the crosspiece and pin linked to it axially forwardly relative to the key body in order to extend the pilot pin.
The disadvantage of this system is that fitting of the pilot pin into the pilot hole is a relatively difficult job. The key body is placed against the chuck with its teeth meshed with those of the chuck and the pilot pin aligned with the pilot hole, whereupon the crosspiece is pressed axially forwardly to extend the pilot pin into the pilot hole. If the pilot pin and pilot hole is not perfectly aligned the chuck key must be maneuvered around to aligned the two and fit them together. Furthermore during rotation of the chuck key it is necessary to continue the axially forwardly directed force on the crosspiece in order to insure that the pilot pin remains projecting axially forwardly out of the chuck key.